Better logging, new preview.lua plugin.

When an exception is caught, info will be printed to the config.log_chat or the console.
/preview will give an "unlinked" preview for the link.
youtube.lua now uses config.google_api_key.
youtube.lua now uses unlinked previews.
lastfm.lua gives more informative error messages.
New utility: handle_exception().

config.lua

@@ -12,6 +12,7 @@ return {
 	cli_port = 4567,
 	admin = 00000000,
 	admin_name = 'John Smith',
+	log_chat = nil,
 	about_text = [[
 I am otouto, the plugin-wielding, multi-purpose Telegram bot written by topkecleon.